In the production of optical fibers, a resin coating is applied immediately after spinning molten glass fibers for protection and reinforcement. A known structure of the resin coating has been a double-layered coating structure consisting of a primary coating layer of a flexible resin which is coated on the surface of optical fibers and a secondary coating layer of a rigid resin which is provided over the primary coating layer. A so-called optical fiber ribbon has been known in the art in the application of optical fibers provided with such a resin coating. The optical fiber ribbon is made from several optical fibers, e.g., four or eight optical fibers, arranged on a plane and secured with a binder to produce a ribbon structure having a rectangular cross section. A resin composition for forming the primary coating layer is called a soft coating, a resin composition for forming the secondary coating layer is called a hard coating, and a material for binding several optical fibers to produce the optical fiber ribbon structure is called a ribbon matrix material. Often, the fibers for identification purposes will be further coated with an ink, which is a curable resin comprising a colorant (such as a pigment and/or a dye), or the secondary coating may be a colored secondary coating (i.e, comprise a colorant). In addition, a material for the further binding of several optical fiber ribbons to produce multi-core optical fiber ribbons is called a bundling material.
Characteristics required for curable resins used as coating materials for optical fibers include: being a liquid at room temperature and having a sufficiently low viscosity for excellent coating; exhibiting superior storage stability and no compositional distribution as a liquid; providing good productivity owing to a high cure speed; having sufficient strength and superior flexibility after curing; exhibiting very little physical change during wide range temperature changes, in particular primary coatings should have very low Tg; having superior heat resistance and superior resistance to hydrolysis; showing superior long term reliability due to little physical change over time; showing superior resistance to chemicals such as acids and alkalis; absorbing only a small amount of moisture and water; exhibiting superior light resistance; exhibiting high oil resistance; producing little hydrogen gas which adversely affects optical fibers; and the like.
In the production of optical fibers and optical fiber assemblies, one of the limitations on how fast the production line can be operated is the cure speed of the coatings and/or binder. Accordingly, it is desirable to develop coatings and/or binders with faster cure speed.
The resin composition for coating optical fibers must remain a liquid during production of the coating, and after being stored for a long period of time. If the resin composition solidifies entirely or partially during storage by (e.g.) flocculation or crystallization, the composition must be heated to avoid any problems in the coating process of the optical fibers, thereby impairing handling of the resin composition.
An object of the present invention is to provide a liquid curable resin composition which exhibits a faster cure speed and better aging characteristics, for example, lower yellowing of the cured composition. A further object of the present invention is to provide coatings having a low Tg (glass transition-temperature).